Long-standing tensions between some Trump supporters and demonstrators who say the billionaire is abetting racism with his statements on Mexicans, Muslims and other groups appeared to come to a head on Friday in Chicago.

The Republican presidential front-runner abruptly canceled a campaign rally at the University of Illinois at Chicago after hours of protests "after meeting with law enforcement has determined that for the safety of all of the tens of thousands of people that have gathered in and around the arena," according to a campaign statement.

It was a big win for demonstrators, who finally shut down one of the increasingly raucous rallies. Supporters were less enthused.

Police had been watching and waiting for hours at the proudly diverse university as demonstrators amassed in opposition to Trump. Dozens of faculty and staff had petitioned university administrators to cancel the rally over concerns about a potentially "hostile and physically dangerous environment."